1. Field of the Invention
This invention relates to a discharge lamp, and more particularly to a cold cathode discharge lamp with lower power consumption.
2. Description of the Related Art
Discharge lamps, which account for about half of the illumination sources, are a very important technical field from the viewpoint of industry and people's everyday lives. Recently, a sharply increasing number of cold cathode discharge lamps have been produced as backlight sources for liquid-crystal displays.
One known cold cathode discharge lamp is a cold cathode fluorescent lamp. In this lamp, two cold cathodes are placed in a glass tube in such a manner they face each other. In the glass tube, a noble gas and a trace amount of Hg are sealed (for example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-274156). A high voltage is applied between the two cold cathodes, thereby starting a discharge between the two electrodes. Sustaining the discharge causes ultraviolet rays to be emitted as a result of the excitation of mercury, thereby causing the fluorescent material to emit light. Another known cold cathode discharge lamp is a barrier cold cathode discharge lamp. In this lamp, an electrode is provided outside the tube where a discharging space is formed. The electrode does not make direct contact with the discharging face (for example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-236083).
Such cold cathode discharge lamps have the advantages that the breaking of the heating filament and the consumption of the electron-emitting emitter are less and the service life is much longer than conventional hot cathode fluorescent lamps. For this reason, they have found an increasing number of applications in the field of illumination for industrial apparatuses whose light sources are difficult to replace. A sharply increasing number of cold cathode discharge lamps have recently been produced particularly as backlights for liquid-crystal displays. On the other hand, lamps of the cold cathode type are at a disadvantage in that they have a lower light-emission efficiency than those of the hot cathode type. If the light-emission efficiency is increased, they will be able to replace the existing fluorescent lamps in the field of illumination not only for industrial apparatuses but also for general-purpose apparatuses.
To improve the performance of a cold cathode discharge lamp, the inventors have devised a cold cathode discharge lamp using diamond as an electron-emitting material for cathodes as disclosed in Jpn. Pat. Appln. KOKAI publication No. 2002-298777 and No. 2003-132850. Since diamond has a high electron emission efficiency and a high sputter resistance, a discharge lamp with a high light emission efficiency and a long service life can be provided. The technique for using diamond for cold cathodes and emitting electrons in a vacuum has been already established (refer to, for example, K. Okano, et al., “Low-threshold cold cathodes made of nitrogen-doped chemical-vapour-deposited diamond,” Nature, Macmillan, 1996, Vol. 381, p. 14).
Another example of using diamond for discharging is a plasma display panel (for example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-117771). In this reference concerning a plasma display panel, covering the surface of diamond particles with an amorphous carbon layer improves the electron emitting characteristic and the light emission efficiency.
In each of the cold cathode discharge lamps, applying a specific voltage to the electrodes causes a discharge to start in the discharge tube. Then, the discharging state is sustained, thereby emitting light. Conventional cold cathode discharge lamps have disadvantages in that the voltage applied to start discharge and the voltage to sustain the discharging state are not low and therefore the power consumption is high. Therefore, the realization of a discharge lamp with a higher light emission efficiency and a lower power consumption has been desired.